The present invention relates to electron microscopes and more particularly, to an electron microscope capable of automatically adjusting the focus of an electron lens and astigmatism of an electron beam.
Conventionally, in observing, measuring, analyzing and searching a field of view of an enlarged specimen image with an electron microscope, an operator participates in performing focusing and astigmatism adjustment while directly watching the enlarged specimen image. In adjusting focus and astigmatism, while the focus correction is carried out in the direction of height of a specimen (referred to as Z direction), the astigmatism correction is carried out in respect of astigmatic aberrations in 0° and 90° directions (X direction) of the specimen and in respect of astigmatic aberrations in 45° and 135° directions (Y direction) of the specimen and therefore adjustments in three directions in total must be accomplished. Namely, when only the focus correction is completed but any astigmatism remains, the enlarged specimen image becomes blurred, resulting in degradation of image quality and inaccuracy of analytical position. It has been practice that the operator participating in the focus and astigmatism adjustments identifies a focus offset and an astigmatic offset while watching an enlarged specimen image. Generally, for the focus correction, excitation current of an objective lens coil serving as an electron lens is changed and for the astigmatism correction, excitation current is supplied to a stigmator coil.
A method for automatically performing focusing is disclosed in, for example, JP-A-61-281446, according to which the focus correction is carried out by deflecting and tilting an electron beam, failing to perform the astigmatism correction and besides the electron beam is so conditioned as to transmit through the specimen.
A focusing and astigmatism adjustment method using a CCD camera is discussed in Ultramicroscopy 49 (1993), pp95-108 but in the method, the specimen is limited to amorphous thin films and besides, operational magnification is limited to high magnification at which the amorphous structure explicitly exhibits itself.
Also, in JP-A-2001-68048 and Japanese Patent No. 3021917, a method for automatic focusing and astigmatism correction is disclosed, which gives a description of astigmatism correction operation but fails to quantitatively determine an astigmatic difference amount and quantitatively decide steady completion of astigmatism correction, thus relying on the level of operational skillfulness of operator.